1. Field of the Invention
The present invention relates generally to a Time Division Multiple Access (TDMA) communications apparatus for performing TDMA communication in which each frame is divided into a plurality of time slots and every participating station transmits information in allocated slots. More particularly, the invention pertains to slot allocation for a TDMA communications apparatus.
2. Description of the Related Art
In a TDMA communication system, each successive frame is divided into a plurality of time slots based on a common time reference (time base) obtained from the global positioning system (GPS), for example, according to a prescribed time division scheme to thereby establish accurate slot synchronization among a plurality of participating stations (e.g. mobile units such as ships). The participating stations carry out TDMA communication while reserving (allocating) slots for their own transmission of information and messages. In this kind of communication system, such as the Universal Automatic Identification System (UAIS), it is essential for every participating station to communicate data in accordance with a defined protocol. To enable proper slot allocation for transmission of information and messages by multiple stations in the UAIS, international and national standards set out detailed technical operational rules and specifications including Self-Organized Time Division Multiple Access (SOTDMA).
Each time a participating station transmits a message (data) in an allocated time slot, the station also announces a slot number indicating a time slot to be reserved for future use (next transmission) by the same station according to SOTDMA protocol. This enables each station to know scheduled future transmissions, or allocated slot numbers for future use, by other participating stations, so that the occurrence of transmit slot allocation conflicts could be avoided.
When transmitting a message in a slot in a particular frame, a participating station can announce a time-out value together with a slot number to be reserved for future transmissions. The time-out value thus announced, which is normally selected from a range of “3” to “7”, is a preliminary indication of the station's intention to use the same slot in several frames succeeding to the current frame. Provided that one station selects and announces a time-out value “4” when transmitting a message in a particular slot in the current frame, for example, that station successively reserves the same slot in the four succeeding frames while decrementing the time-out value from “4” to “3”, “21”, “1” and “0”. More specifically, if the station has announced the time-out value “4” when transmitting a message in a slot in a first frame, the station announces the time-out value “3” and definitely reserves the same slot in a third frame when transmitting a message in a second frame. Next, the station announces the time-out value “2”, and definitely reserves the same slot in a fourth frame when transmitting a message in the third frame. Then, the station announces the time-out value “1” and definitely reserves the same slot in a fifth frame when transmitting a message in the fourth frame. Finally, the station announces the time-out value “0” when transmitting a message in the fifth frame.
In the aforementioned slot allocation scheme of the SOTDMA protocol, a TDMA communications apparatus preliminarily reserves, or pre-reserves, the same slot in as many successive frames as specified by the time-out value and definitely reserves, or allocates, that slot at a point of transmission in the slot in each successive frame while decrementing the time-out value by 1. As the TDMA communications apparatus of each station decrements the time-out value one by one in this fashion, all other stations can recognize for how many frames the relevant slot should not be allocated for their own use referring to the time-out value transmitted from that TDMA communications apparatus. If the time-out value of one station is “3” in a particular slot in a current frame, for example, the other stations should refrain from allocating that slot for their own use for consecutive four frames including the current frame. At a point in time when the time-out value has reached a specific value (“0” in the above discussion), the TDMA communications apparatus selects and announces a desired slot number specifying a slot to be pre-preserved for future transmissions and announces that slot number together with a new time-out value when transmitting a message as described in Japanese Patent Application Publication No. 2003-057579, for example.
According to the SOTDMA protocol currently used in the UAIS system, however, there is a possibility that a transmit slot allocation conflict (overlapping of allocated slots) may occur even when a sufficient number of unreserved slots are available and conditions for radio communication are favorable. Such a slot allocation conflict situation is explained below more specifically, by way of example.
FIG. 6 is an explanatory diagram showing a transmit slot allocation conflict which can occur under the currently used SOTDMA protocol.
Referring to FIG. 6, if station A and station B transmits in two successive slots (e.g., slot No. 100 and slot No. 101) in a particular frame and the time-out value reaches a specific value (“0” in a case where the time-out value is decremented from one frame to the next as stated above) on both stations A, B in the same frame, there is a possibility that a slot number specifying a slot to be pre-preserved for future transmissions by station A coincides (conflicts) with a slot number specifying a slot to be pre-preserved for future transmissions by station B.
More specifically, in order to transmit in slot No. 100, a transmitter section of station A edits a transmit message including a slot number (slot No. 99 in frame c as illustrated in FIG. 6) to be pre-preserved for future transmissions and a time-out value. Then, station A transmits the message to station B and other participating stations in slot No. 100.
On the other hand, a receiver section of station B receives the message transmitted by station A in slot No. 100 and analyzes the message in slot No. 101. Therefore, station B recognizes the next slot number (slot No. 99 in frame c as illustrated in FIG. 6) to be allocated to station A at a point in time in slot No. 101. In order to transmit in slot No. 101, a transmitter section of station B edits a transmit message including a slot number (slot No. 99 in frame c as illustrated in FIG. 6) to be pre-preserved for future transmissions and a time-out value. Then, station B transmits the message the other participating stations in slot No. 101.
It will be understood from the above discussion that station B may select and announce a slot number (slot No. 99 in frame c as illustrated in FIG. 6) specifying the slot to be pre-preserved for future transmissions before knowing that station A has pre-preserved the same slot for future transmissions. Should such a situation occur, the slot number allocated to station A conflicts with the slot number allocated to station B.